Unlimited number of a monster:
Note: This trick may take some time. Decide what monster you want. They are in the following order: Suezo, Mocchi, Hare, Tiger, Golem, and Gali. Make sure that you do not get the bonus stage in stage 6. Go through the game finding the bonuses until you get the monster just before the one you want. Then, go to the bonus stage in stage 6 to catch the monster just after the last one you caught (according to the list). Keep restarting the tower and going to stage 6 repeatedly until you get as many as needed. As long as you also collect the mystery disk pieces you should not run out of them.